var span = document.getElementsByTagName('span')[0];
var mark = document.getElementById('mark');

document.body.onclick = function(e){
    e = e || window.event;
    var curDisplay = window.getComputedStyle(mark,null).display;
    if(e.target.tagName.toLowerCase() == 'span'){
        if(curDisplay == 'none'){
            mark.style.display = 'block';
        }else if(curDisplay == 'block'){
            mark.style.display = 'none';
        }
    }else if(e.target.id == 'mark'){
        return;
    }else{
        mark.style.display = 'none';
    }

    
}